This invention relates to semiconductor devices having a mesa structure, and particularly to the configuration and fabrication of the mesa structure.
The present invention is an improvement of those disclosed in my U.S. Pat. Nos. 4,740,477, 4,891,685 and 5,010,023, the subject matter of which is incorporated herein by reference.
These patents show rectifier devices comprising a mesa structure, particularly a frustum shaped mesa having a circular cross-section. The mesa contains various doped regions including an N.sup.+ N.sup.- junction which is generally parallel to the base plane of the mesa and which intercepts the side wall of the mesa. At the side wall, the plane of the N.sup.+ N.sup.- junction curves slightly upwardly. As explained in the patents, the combination of a sloped mesa wall and a curved N.sup.+ N.sup.- intercept therewith results in the voltage breakdown characteristics at the side of the mesa being improved, whereby problems associated with voltage breakdowns at surface intercepts of voltage blocking p-n junctions are reduced.
The theory concerning surface voltage breakdown is presented by O. Melville Clark in U.S. Pat. No. 3,260,634 (Jul. 12, 1966) and by R. L. Davies, et al, in "Control of Electric Field at the Surface of P-N Junctions," IEEE Transactions on Electron Devices, July, 1964, pp. 313-323. The Davies article describes the use of mesas having sloped side walls. In the devices experimented with by Davies et al, sloped walls were obtained by mechanical means, e.g., grinding. However, in semiconductor devices using modern manufacturing techniques, the use of various mechanical means for providing mesa structures is quite impractical.
In my above cited patents, conventional photolithographic techniques are used involving anisotropic etching using a circular mask. The result, as shown in my patents, are frustro-conical mesas. A problem, however, is that using known anisotropic etching techniques, the actual slope of the mesa wall varies rather significantly from point to point around the periphery of the conical mesa. In general, the mesa conical wall has a slope of around 45 degrees, as measured from the mesa base plane, which is generally desirable, but the wall includes, as an inherent result of the anisotropic etching process, four bulges, disposed at 90 degrees from one another around the mesa periphery, where the wall slope increases to around 54 degrees. These variations in mesa wall slopes tend to provide non-uniform device characteristics around the periphery of the mesa and must be taken into account in the design of the rectifier devices. In general, better devices, more easily mass produced, result if the slope variations, at least in frustro-conical mesas, are not present or greatly reduced in size.
My patents also mention the use of a mesa having a generally square cross-section with rounded corners. As described hereinafter, such a square-shaped mesa does indeed provide improved results in accordance with the present invention, but with certain requirements not known to me at the time of filing of the applications resulting in my above-cited patents and only recently worked out by me after extensive observations and analyses.